We propose to examine the structures and properties of classical and nonclassical nucleic acid base pairs using theoretical methods. One specific aim of this research is to study the molecular structures and stabilities of the above compounds by ab initio quantum mechanical calculations. We shall study hydrogen-bonding and stacking interactions between classical purine nucleic acid bases and classical pyrimidine nucleic acid bases; as well as non-classical systems that might increase the catalytic role of RNA and DNA molecules and the fidelity of replications. We shall predict interaction energies of the WC and reverse-WC pairs of classical and nonclassical species and their stacking complexes. Different derivatives of the classical NABs will be examined to gain an understanding of how selected substituents could modify the probability of spontaneous mutations of the parent molecules. We shall assist experimental studies on the title species by providing theoretical molecular geometries, dipole moments, rotational constants, and vibrational frequencies and intensities. We shall expose underrepresented minorities to the theory and practice of quantum chemical methods. The development of computer and supercomputer literacy and of the oral and written communicative skills of students will be a major thrust of this research project.